Sub process challenges in ultra fine pitch stencil printing of type‐6 and type‐7 Pb‐free solder pastes for flip chip assembly applications

Abstract

PurposeThe study investigates the sub process behaviour in stencil printing of type‐6 and type‐7 particle size distribution (PSD) Pb‐free solder pastes to assess their printing limits.Design/methodology/approachTwo solder pastes were used in a design of experiments approach to find optimal printing parametersFindingsSolder paste printing has been achieved to ultimately produce 30 μm deposits at 60 μm pitch for full area array patterns using a type‐7 Pb‐free solder paste. For a type‐6 PSD solder paste, full area array printing was limited to 50 μm deposits at 110 μm pitch. However, for peripheral printing patterns, 50 μm deposits at 90 μm pitch were obtained. The disparities in the behaviour of the two paste types at different geometries can be attributed to differences in the sub‐processes of the stencil printing. The paste release of the type‐6 paste from the stencil apertures at fine pitch was superior to the type‐7 paste, which may be attributed to the finer particle paste producing an increased drag force along the stencil aperture walls. However, the type‐7 paste was able to fill the smallest aperture openings, ultimately to 30 μm, thus producing full array printing patterns at uniquely small pitches.Practical implicationsThis advancement in the stencil printing process has been made possible by refinements to both solder paste design and stencil manufacturing technology. Adjustments in the solder paste rheology have enabled successful printing at ultra fine pitch geometries. This, together with selecting appropriate printing parameters such as printing speed, pressure, print gap and separation speed, allows a practical printing process window. Moreover, advancements in stencil fabrication methods have produced “state‐of‐the‐art” stencils exhibiting very precisely defined aperture shapes, with smooth walls at very fine pitch, thus allowing for improved solder paste release at very small dimensions.Originality/valueThe results can be used to present a low cost solution for Pb‐free flip chip wafer bumping. Furthermore, the results indicate that type‐6 and type‐7 solder pastes should be applied to/selected for specific application geometries.

A hybrid intelligence technique based on the Taguchi method for multi-objective process parameter optimization of the 3D additive screen printing of athletic shoes

“…The ITR and PQS were both the LTB. So the ITR-S/N and PQS-S/N were calculated using equations (2) and (4). Tables 5 and 6 and Figures 7 and 8 show the main effects of S/N ratios on responses.…”

“…3 Studies have shown that the printability of the ink is also important. 4,5 However, many research results are not practical and can't play a guiding role in the actual printing production process. This is because these studies involve complex mathematical models which are difficult for the process practitioner and they need to use some special instruments, which are expensive.…”

A review of stencil printing for microelectronic packaging

“…In 2003, a refined version of the DC electroformed stencil manufacturing process was described (Kay et al , 2003). This stencil was manufactured using the UV‐LIGA process alongside bipolar electroforming (Kay and Desmulliez, 2003; Jackson et al , 2005). The process can produce stencils with apertures having micron tolerance and smooth sidewalls, hence giving excellent paste release.…”

“…Particle size distribution (PSD) for solder paste is separated into eight main types as shown Table I. Currently, types 7z and 8 solder pastes are primarily development materials used in producing fine pitch flip‐chip interconnects (Jackson et al , 2005; Manessis et al , 2006).…”

Wire bonding using copper wire

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